Ungulate Herbivory Is Correlated with High Aspen Suckering Density but Reductions in Aspen Growth Rates and Recruitment

Aspen is a foundation species that provides vital habitat for hundreds of plant and animal species. However, changing ungulate herbivory regimes may be altering recruitment success and resilience of aspen forests. The objective of this observational study was to quantify the impacts of ungulate herbivory on aspen recruitment potential. We sampled 36 aspen dominant stands on the Fishlake National Forest in Utah for browse of apical meristems, suckering density (< 180 cm in height), and recruitment density (≥ 180 cm). Our analysis indicated that for each 10% of apical meristems browsed, annual vertical and radial growth rates and recruitment density were reduced by 1.2 cm, 0.02 mm, and 17 ha^? 1, respectively. In contrast, aspen suckering density increased by 1 230 suckers ha^? 1, with each 10% browse of apical meristems. This suggests that ungulates contribute to aspen suckers persisting in the understory for longer periods and may alter recruitment success of aspen stands. Aspen that were under high ungulate browse pressure were associated with lower growth rates and persisted for twice as long in the understory. For example, a 5-yr-old sucker was predicted to be 115 cm without browse of apical meristems and 60 cm with 100% browse of apical meristems. Further, aspens of the same height with 100% browse of apical meristems were nearly 4 yr older on average than aspens with no evidence of browse of apical meristems. Approximately 34% of aspen suckers had persisted in the understory < 100 cm for ≥ 6 yr, and 7% of suckers had persisted in the understory for ≥ 10 yr. Our results suggest that high rates of meristem removal are correlated with increased aspen suckering yet reduced aspen growth and recruitment.     

Cattle Selection for Aspen and Meadow Vegetation: Implications for Restoration

There is concern over the decline of aspen and the lack of successful regeneration due to excessive browsing of aspen suckers by cattle and other wild and domestic ungulates. We conducted a 2-yr study on Lassen National Forest, California, to aid development of cattle grazing strategies to enhance aspen regeneration. We evaluated seasonal biomass, nutritional quality, and utilization by cattle of aspen suckers, aspen herbaceous understory vegetation, and meadow herbaceous vegetation within six aspen–meadow complexes. Aspen suckers had greater nutritional quality compared to aspen understory and meadow vegetation regardless of season or year. Nutritional quality declined with season in all three vegetation types. Early-growing season foraging by cattle focused on meadow and aspen understory vegetation. Mid-growing season decreases in meadow and aspen understory nutritional quality coincided with a marked increase in utilization of aspen suckers. By late-growing season, utilization on aspen suckers was significantly greater than aspen understory or meadow vegetation. Managers can use early-growing season grazing to reduce aspen consumption by cattle, set stocking rates so that adequate herbaceous vegetation is available throughout the growing season, provide nutritional supplements to reduce demand for nutritious aspen suckers, construct protective fencing, and implement grazing systems that insure years with mid- and late-growing season rest from heavy browsing.     

Extent of Stem Dieback in Trembling Aspen (Populus Tremuloides) as an Indicator of Time-since Simulated Browsing

Simulated browsing treatments were imposed on an important browse species of the North American moose (Alces alces L.) to see if the development and extent of subsequent stem dieback in trembling aspen (Populus tremuloides Michx.) could be used to determine the time of browsing during the growing season. Two hundred naturally growing aspen saplings of similar size and form were randomly selected in a 20-ha area near the endowment lands of the University of Northern British Columbia, Prince George, British Columbia, Canada. Plants were randomly assigned to treatment categories so that the apical meristems of 50 plants each were assigned to a control or were clipped on one of the following dates 6 weeks apart: 1 June, 16 July, and 30 August 2005. The leader of each aspen was clipped and dieback was left to progress until the onset of winter dormancy. Our results showed that the earlier the simulated browsing occurs in the growing season, the greater the length of stem dieback, up to the maximum of the subapical axillary node below the point of clipping. The average rate at which dieback progressed varied between treatments and decreased throughout the growing season. Our results suggest that the ratio of the actual length of stem dieback to the overall length of stem between the clip point and the subapical axillary node serves as a good indicator for estimating the time at which aspen meristems have been browsed during the growing season.     

Effect of Conifer Encroachment Into Aspen Stands on Understory Biomass

Conifers (Picea and Abies spp.) have replaced aspen (Populus tremuloides Michx.) over much of aspen’s historic range in the western United States. We measured the impact of this change upon the production of understory vegetation potentially useful as forage for livestock and wildlife on two southern Utah national forests. A negative exponential relationship between conifer cover and understory biomass was demonstrated as log(biomass) = 6.25 − 0.03787(% conifer), adjusted R² = 0.57. Understory production in aspen stands begins to decline under very low levels (10% to 20%) of conifer encroachment. Management implications include loss of forage production capability and wildlife habitat and potential overstocking of livestock grazing allotments if the associated loss of forage is not considered.     

Behaviour of goats, sheep and cattle on natural pasture in the sub-humid zone of West Africa

The behaviour of sheep, goats and cattle on a shrub and tree savannah in the sub-humid zone of West Africa was studied during the dry (February to May), rainy (June to September) and cool season (October to January). Two mature females per animal species were followed by two observers during 3 days per month and behaviour activities were recorded every 15 min of the daily grazing time. When browsing, the plant name, plant part and plant height reached during prehension were recorded. The distance travelled and the grazing circuits were recorded with a step counter and a Geographic Position System device. The knowledge of farmers concerning appreciated browse species relative to the browse species consumed by animals in the area was also investigated.

During the dry season sheep, goats and cattle spent around 25% of their time browsing and 20%, 16% and 31%, respectively, of their time grazing. During the cool season, goats (17%) browsed more than sheep (7%) and cattle (5%) (P < 0.05). During the rainy season, sheep browsed 20% of the time while cattle spent only 7% of their time browsing, while goats were tied, with no opportunity for browsing. Browsing leaves was the most important part of the browsing activity. During the cool season, browsing height was the same (1.0 m) for cattle and goats, but lower for sheep (0.6 m) (P < 0.05). In the dry and cool season, sheep and goats walked shorter distances than cattle.

The plant species with the highest frequency of consumption were Acacia dudgeoni Craib, Ficus gnaphalocarpa (Miq.) A. Rich, Dioscorea togoensis Knuth, Cordia myxa L., and Manguifera indica L. for sheep and goats and Afzelia africana Sm., Kaya senegalensis (Desv.) A. Juss, Pterocarpus erinaceus Poir. and Dicrostachys cinerea (L.) Wight and Arn. for cattle. Species with a high frequency of citation by farmers were A. africana (92%), K. senegalensis (88%), P. erinaceus (86%), Faidherbia albida A. Chevalier (80%), A. dudgeoni (70%) and F. gnaphalocarpa (69%).

In conclusion, browse species make an important contribution to the diet of ruminant animals in the sub-humid zone and farmers are familiar with the species consumed by ruminant animals.     

Research note: Tests for inducible thorn defences against herbivory must consider plant growth strategies

Abstract

Thorn length and density on the latest year's growth were compared at three height (feeding) levels (<1m, >1m<2 m and >2 m) on individuals of Acacia karroo and Ziziphus mucronata in an area that was browsed by goats. Thorn length and density was greatest for branches within easiest reach of goats. However, there was no increase in thorn length or density on the latest year's growth of branches of A. karroo individuals protected from browsing. Thorns on distal ends of branches are relatively small and sparse and this is partly due to differences in internode length. Herbivores limit the extent of annual growth and this prevents development of small thorns characteristic of late season growth. Thus, browsed individuals which lack late season growth may have greater spinescence. This is only a passively induced response and is not due to the active production of larger thorns subsequent to browsing.     

The Effect of Supplementation with Fresh Browse of Ziziphus mauritiana or Combretum aculeatum on Feed Intake,Nitrogen Utilization and Growth of Grazing Range Sheep

The influence of 0, 60 or 120 min access to a grove of either Ziziphus mauritiana (Ziziphus) or Combretum aculeatum (Combretum) on forage intake and on the digestibility, growth and excretion of nutrients was determined using 40 Oudah rams grazing for 7 h/day on poor-quality dry season pasture in the Sahel. The effects of browse species and browsing duration (0, 30, 60 or 120 min) on the ruminal ammonia content were also evaluated using 8 mature fistulated rams. Ziziphus increased both total digestible organic matter and total dry matter intakes per (kg live weight)^0.75 without decreasing herbage intake, whereas 60 or 120 min access to Combretum reduced herbage intake by 6.3% and 4.2%, respectively. The digestibility of the diet decreased (p<0.05) with the duration of access to the groves. Despite the provision of more nitrogen (N) than in the control diet, ruminal NH₃-N decreased 24 h after browsing commenced. The reduced live weight gain of sheep browsing Combretum may indicate more deleterious compounds in Combretum than in Ziziphus. Ziziphus appears to have more potential than Combretum to increase sheep production in low-input crop/livestock systems.     

Aspen Recovery Since Wolf Reintroduction on the Northern Yellowstone Winter Range

Quaking aspen (Populus tremuloides Michx.) recruitment and overstory stem densities were sampled in 315 clones in 1991 and 2006 on 560 km² of the Northern Yellowstone Winter Range (NYWR). A primary objective was to observe if aspen status had improved from 1991 to 2006: evidence of a wolf (Canis lupus) caused trophic cascade. Recruitment stems (height > 2 m and diameter at breast height < 5 cm) represent recent growth of aspen sprouts above elk (Cervus elaphus) browsing height, whereas overstory stems (all stems > 2 m) represent the cohort of stems, which will insure the sustainability of the clone. Overstory stem densities declined by 12% (P = 0.04) on the landscape scale when compared with paired t-tests. Overstory stems declined in 58% of individual clones and in 63% of the 24 drainages of the study area. The second objective was to determine which factors influenced changes in aspen density. Winter ungulate browsing (P = 0.0001), conifer establishment (P = 0.0001), and cattle (Bos spp.) grazing (P = 0.016) contributed to the decline in overstory stem densities when analyzed using a mixed effects model of log transformed medians. Eighty percent of the clones were classified as having medium to high browsing levels in 1991, whereas 65% of the clones received a similar rating in 2006, possibly due to the reduced NYWR elk population. Aspen recruitment has increased in some 2–10 km² areas, but not consistently. Our study found that a trophic cascade of wolves, elk, and aspen, resulting in a landscape-level recovery of aspen, is not occurring at this time.     

光强和刈割处理对柠条幼苗补偿生长的影响

在3个光强,100%自然全光照(natural sunlight,NS)、37%NS和16%NS的遮阴条件下,研究了5个刈割强度处理(留茬高度为30、20、10、3cm和不刈割)对柠条(Caragana korshinskii)幼苗补偿生长的影响。结果表明,1)强光照和刈割均促进幼苗刈割后的补偿生长,刈割处理后幼苗的萌条数和萌条基径都在强光下最大,但随着刈割强度的增大呈减小趋势。2)幼苗株高、基径、主根长和侧根数在所有刈割处理均表现为100%NS37%NS16%NS;刈割处理幼苗的株高和基径均小于不刈割对照,且随刈割强度的增大而减小;尽管轻度刈割促进幼苗主根生长和侧根数增加,但二者都随刈割强度的增大而受到抑制。3)强光照有利于幼苗刈割后的生物量积累,但幼苗总生物量在不同光照条件下都随刈割程度的增大逐渐减小,即表现为欠补偿生长;幼苗生物量补偿指数随着光照强度的减弱而增大,在16%NS下不同强度刈割处理间变化不大,表明幼苗刈割后的生物量积累在强光下主要依赖于光合作用新合成的光合产物,而在弱光下主要依赖于根系和残留树桩贮藏的营养物质。研究表明,柠条幼苗刈割后的补偿生长能力随着刈割强度的增大而减小,且表现为欠补偿;强光照对柠条幼苗刈割后的补偿生长具有促进作用。本研究可为柠条的幼苗培育、饲草生产和草地生态系统管理提供参考。     

高寒草甸土壤物理特性和微生物数量对模拟践踏的响应

为探明高寒草甸土壤对家畜践踏的响应机制,于甘肃天祝进行了牦牛和藏羊的模拟践踏试验,研究了不同强度模拟践踏对土壤物理特性及微生物数量的影响。结果表明,0～10 cm土层的土壤紧实度和容重,随着践踏强度的增加均呈递增趋势;而呼吸速率和饱和导水率呈递减趋势。0～10 cm土层的3大微生物数量随践踏强度的增大而减小。同一践踏强度下,牦牛践踏后0~10 cm土层的土壤紧实度和容重均大于藏羊践踏处理;而饱和导水率、呼吸速率和微生物数量均小于藏羊践踏处理。综合分析认为,家畜的践踏强度越大,土壤的压实效应越显著;一个生长季的模拟践踏主要影响0～10 cm土层土壤的物理性质;且牦牛对土壤的压实效应和对微生物分布的抑制作用均大于藏羊。     

Browsing of Grewia occidentalis by domestic stock in a South African savanna: the influence of bush clumps

In South African karoo categorised by multi-species bush clumps, skirted shrub species appear to physically defend the palatable shrub Grewia occidentalis against browsing by eland (Taurotragus oryx). Skirted shrubs appear to defend G. occidentalis by preventing access to plants and by reducing the accessibility of G. occidentalis foliage to the large muzzle of eland. Compared to eland, domestic stock such as goats, have a smaller body and muzzle, and these features may allow them to overcome the physical defence offered by skirted shrubs against the browsing of G. occidentalis. Our study of G. occidentalis browsed by domestic stock indicated that mature and seedling G. occidentalis typically occurred with skirted shrub species. Mature and seedling G. occidentalis growing with skirted shrubs had lower intensities of browsing than other G. occidentalis, and mature G. occidentalis growing with skirted shrubs had greater numbers of fruit than other G. occidentalis. In our study area, a non-selective grazing/browsing system is used and camps are grazed/browsed until a high proportion of forage is removed from most plants. Under this system, mature and seedling G. occidentalis are common within the study area and skirted shrubs appear to provide adequate defence for G. occidentalis against browsing by domestic stock.     

Initial Effects of Brush Cutting and Shoot Removal on Willow Browse Quality

We examined the initial effects of brush cutting (removal of all aboveground biomass), as well as clipping (removal of current annual shoots) and ungulate browsing (collectively referred to as shoot removal) on the morphology and nutrient quality of Scouler's willow (Salix scouleriana J. Barratt ex Hook.) for ungulates on sites 2 and 4 years after brush cutting. We specifically assessed changes in the biomass, tannin content, digestible energy, and digestible protein of shoots from brush-cut willows relative to shoots of uncut willows to determine how browse plants respond to this form of vegetation management. In winter, the resprouted current annual shoots of willows that had been brush cut were larger in mass and lower in digestible protein than shoots of uncut willows for at least 4 years after brush cutting. Shoots of brush-cut willows were also lower in tannin and digestible energy than the shoots of uncut plants for two winters after brush cutting. In the second winter after brush cutting, shoot biomass decreased and tannin content increased with increasing shoot removal during the previous winter. In the fourth winter after brush cutting, shoot mass increased and digestible energy decreased in shoots with greater shoot removal. Nutrient quality was otherwise unaffected by the amount of shoot removal during the previous winter. Because of the occasional importance of site effects in this study, we recommend that long-term studies maximize the number of sampled sites. Because brush cutting alters the quality of regenerating browse and can affect how ungulates utilize such browse for several years after brush cutting, we further recommend that forest vegetation managers consider potential impacts of brush cutting on ungulate winter range.     

Nutritional composition of three fodder species browsed by camels (<Emphasis Type="Italic">Camelus dromedarius</Emphasis>) on arid area of Tunisia

Browsing camels have access to different plant species in relation to the kind of pasture they can reach. A study was conducted in an arid region of Southern Tunisia to determine the dietary preference and forage quality of free ranging camels. Foliage consisting of leaves from Limonium pruinosum, Retama raetam and Stipa tenacissima, was collected during the dry season to evaluate the chemical characteristics and nutritional value of these browse fodder species. The dietary preference was studied using 15 adult camels which were selected from a herd of 50 animals appropriately marked for identification. There was a significant difference in the chemical composition and nutritional value of plant species collected. Based on crude protein (CP) content and nutritional value, the three fodder species browsed can be recommended as good-quality food source for camels under pastoral management.     

Moose Summer Diet From Feces and Field Surveys: A Comparative Study

Microhistological analysis of feces is the most applied noninvasive method for assessing diets of wild ungulates. However, the method is complicated by differential digestibility of forage species. To evaluate the efficacy of this method in quantifying browse components in summer diets of moose (Alces alces L.) on Norwegian rangelands, we compared it to parallel field surveys of browsed vegetation on the same range. Although the same principal diet components were identified in the feces and in the field, there were consistent discrepancies between the two methods in estimated proportional diet contents. Birch (Betula spp.) showed the highest field:fecal ratio: 3.3 ±  compared to 0.9 ±  for Salix spp., 0.8 ±  for aspen (Populus tremula L.), and 0.6 ±  for rowan (Sorbus aucuparia L.). Until in vivo fecal correction factors for differential forage digestibility are available, we caution against broad application of fecal analyses for estimating proportions of browse in moose diet. Although we could not determine the exact amount of discrepancy implicit in each method, previous studies of moose summer diet in the area clearly indicate that fecal analyses gave a less accurate representation of actual moose browse diet than did the field survey. Fecal analyses are nevertheless needed to identify moose diet components other than browse, which are not easily obtained from field surveys.     

Variation in quantity and quality of native forages and grazing behavior of cattle and goats in Tanzania

The study was conducted to assess the effects of seasonal variation in the quality and quantity of pasture and management of livestock exclosures (ngitili) on the grazing behavior of cattle and goats. The study was 2×2×2 factorial design with three independent variables: season (Dry or Rainy), ngitili management (Private or Communal) and animal species (Cattle or Goats). Focal and scan observation methods were used to record different behavioral activities. Vegetation attributes from the study areas were measured in two consecutive seasons. Most key forage species had significant higher crude protein (CP) content and in vitro organic matter digestibility (INVOMD) in rainy than in dry season (P<0.05), but Neutral Detergent Fiber (NDF) and Acid Detergent Fiber (ADF) did not vary significantly with season (P>0.05). ADF and NDF were significantly higher in species from communal ngitili than those from private ngitili (P<0.05). Above-ground herbaceous biomass and bulk density (BD) were significantly higher in the rainy season and in the private ngitili than in the dry season and in the communal ngitili respectively. Cattle and goats spent considerably more time grazing and browsing respectively in the rainy season than in the dry season (P<0.05). Cattle foraging activities did not vary significantly (P>0.05) with ngitili management, but goats found to spend considerably more time browsing in the communal ngitili and more time grazing in the private ngitili (P<0.05). Despite the merits of stocking cattle and goats together in the heterogeneous pasture, seasonal variation in forage resources requires investigation of other strategies such as use of multipurpose trees and treatment of crop residues to improve livestock production.     

祁连山区黑土滩人工草地对春季模拟放牧的响应

该研究于2018年牧草返青期以刈割模拟家畜采食行为,对祁连山区黑土滩人工草地进行了放牧强度(刈割率)为0、30%、50%、70%、90%的5级模拟放牧试验研究。结果表明,春季休牧(未刈割)和放牧强度(刈割率)为30%时,对黑土滩人工草地垂穗披碱草的后期生长发育没有影响,而随着放牧强度(刈割率)加大,垂穗披碱草的后期生长发育将会受到严重制约,当放牧强度(刈割率)达到90%时,垂穗披碱草的盖度、高度、牧草产量和种子产量分别较休牧降低了41.0%、73.0%、87.4%和88.9%,研究结果认为返青期过度放牧利用会引起黑土滩人工草地的快速退化。     

不同刈割强度下草地群落、层片及物种的补偿性生长

群落中不同层片物种对刈割的响应有所不同,探讨不同刈割强度下群落各层片物种的补偿性生长对提高群落生产力具有重要意义。本试验以水热条件较好的南方草地——重庆云阳岐山草场为研究样地,采用梯度刈割方式(不刈割、留茬10 cm、留茬6 cm),进而研究不同刈割强度下,草地植物群落、层片以及物种补偿性生长的响应。结果发现,1)中度刈割(MC)下群落生物量恢复最快,而重度刈割下群落地上相对生长速率最大;2)生长季末时,中度和重度刈割下地上部分均实现了超补偿生长;3)顶层物种在中度刈割下地上相对生长速率(relative growth rate, RGR)最大,而中层物种在重度下最大;4)中度刈割下,群落的顶层物种生物量与群落生物量显著正相关,重度刈割下,顶层和中层物种生物量与群落整体生物量正相关,底层物种生物量与群落总生物量在两种刈割强度下均不相关;5)物种水平上芭茅、鸭茅和白三叶生物量变化与群落地上生物量变化基本一致。研究结果表明,在草地群落中,不同层片物种对刈割的响应存在差异,中度刈割更利于顶层物种的超补偿生长,重度刈割则有利于中层物种的超补偿生长。因此根据不同层片物种所占群落的比重确定草地刈割强度能更有效发挥草地的增产潜力。     

Feeding ecology of four livestock species under different management in a semi-arid pastoral system in South Africa

The Leliefontein communal area in Namaqualand is grazed by mixed species herds that have multiple benefits for pastoralists. This study assessed how the management (herding and free-ranging) of different livestock herds affects their feeding ecology during the wet and dry seasons in a semi-arid shrubland. By using direct observations of livestock grazing in the field, we established their habitat preferences, diet selection and resources overlap. Results indicate that all free-ranging livestock can be considered grazers with cattle being selective. When sheep are herded, they tend to be more generalist feeders and herded goats are browsers. Livestock diet selection and forage preferences are largely linked to the habitats in which they graze and browse. Dietary overlap is highest between free-ranging livestock irrespective of season, whereas herded livestock are able to consume a food source with little potential competition from other livestock. Although the wet-season annual forage resources are able to sustain the livestock population in Leliefontein, this study concludes that the overdependence on annual vegetation would make livestock vulnerable during drought periods when forage production is low.     

夏季模拟放牧对青藏高原冬春草场牧草品质和土壤养分的影响

为探索青藏高原地区草畜高效管理模式,本试验采用刈割模拟家畜采食的方式在夏季牧草生长季（8月初,称作青草期）利用冬春草场,并在牧草生长季末（10月初,称作枯黄期）采集牧草和土壤样品,研究不同利用强度对牧草再生、生物量、品质以及土壤养分的影响。试验结果表明：3种不同利用强度对牧草枯黄期地上生物量、牧草酸性洗涤纤维（Acid detergent fiber,ADF）和中性洗涤纤维（Neutral detergent fiber,NDF）没有产生显著影响;不同草地利用强度处理组枯黄期粗蛋白（Crude protein,CP）含量均显著低于青草期,而ADF和NDF的结果刚好相反（P<0.05）;枯黄期重度利用草场牧草CP含量显著高于不利用组（P<0.05）,中度利用强度0~10 cm土壤速效磷含量显著高于对照组（P<0.05）。本试验结果表明,青藏高原在牧草生长季利用冬春草场（留茬2~4 cm）,家畜不仅能够摄取优质的青绿牧草,而且不会降低牧草生育期总生物量。